Taxonomic and trophic assembly of the size spectrum in Antarctic plankton

Abstract

Antarctic waters are particularly productive, but its trophic web must evolve under extremely cold temperatures and wide environmental variability at seasonal scale. These special features determine a singular predator community since ectothermic vertebrates are physiologically limited and large-sized en- dotherms, especially those able to accumulate great energy reserves (e.g. emperor penguin) or migrate over long distances (e.g. cetaceans, flying seabirds), find considerable competitive advantages. Although trophic pathways through fish can be regionally significant, their role in Antarctic waters could be considered negligible compared to other similarly productive ecosystems. The lack of planktivorous fish is likely responsible for the successful development of large-sized planktivorous invertebrates. Here, we show how the size classification of the invertebrate organisms in the Antarctic waters get rise to a bumpy biomass distribution with pronounced peaks and troughs, and including exceptionally large organisms. This pattern considerably differs from the size distributions of oligotrophic marine waters but shows strong similarities with the one described for a fish-less temporary lake. Two alternative plankton size spectra were described, one dominated by krill and fueled by large phytoplankton and another one dominated by salps and fueled by small phytoplankton. The biomass accumulations in large size ranges are persistent due to longevity and the complementary wintry foraging strategy of krill. In the case of salps, their short life cycle is balanced with their ability to produce explosive blooms under favorable conditions and their sexual-cycle overwintering strategy. The trophic consequences of a shift from krill to salps dominated Southern Ocean are still unknown but feeding interactions among the main pelagic predators have been discussed to complete a general overview of the Antarctic pelagic realm in the size spectrum.